In the most varied technical fields, numerous cases are known where problems arise of diffusion and solubilizing of a gas phase in a liquid phase. For example, when the liquid is water, it is possible to cite, in a nonlimiting way: injection of carbon dioxide gas in water to be remineralized, with the addition of calcium carbonate; injection of ozone in water to be disinfected; sending of air in activated sludge according to processes of biological treatment of water; the introduction into water to be purified, according to the flotation technique, of a gas having a mechanical role in the use of this technique, etc.
In all these cases the problem arises of obtaining certain dimensions of gas bubbles, which are able to furnish an acceptable transfer coefficient, as well as the rising speed of the bubbles in the liquid, for example in a contact volume (or reactor) constituted, for example, by a bubble column.
Several techniques have been proposed or used to obtain microbubbles of gas intended to saturate an ambient liquid. For example, in the case of water, one of them comprises saturating water with gas (air, oxygen, ozone) in an enclosure kept under pressure then expansion of the water before entry into the contact reactor: thus very fine gas bubbles are obtained with an average diameter of 40 to 100 microns. However, this process requires a considerable expenditure of energy and, further, the microbubbles have a very slow rising speed in the liquid, which can constitute a serious obstacle in some applications.
According to other known processes, an effort is made to obtain a homogeneous series of microbubbles by making a slightly compressed gas diffuse through an element of porous material, for example, with a base of ceramic or sintered metal, whose chosen porosity is a function of the various parameters of application envisaged. However, according to a general phenomenon noted with such systems, the microbubbles have a tendency not to become detached from the porous element from which they have come, after having reached a size of several millimeters, sufficient for the buoyancy to be able to overcome the surface tension forces. They then form a trail of large bubbles which rises very quickly in the contact column, at speeds of at least 20 to 25 cm/s. The dispersion obtained is then quite often insufficient and the loss of gas is considerable.
To improve the transfer efficiencies--gas phase-liquid phase in the context of this diffusion technique by porous material--it has proven advantageous, during injection of pressurized gas under the porous device, to flush the surface of this latter with a liquid current (see, for example, the patents U.S. Pat. No. 3,545,731 and France publication No. 2 421 671).
In water treatment installations, the gaseous fluid is generally made up of oxygenated air, or preferably, of ozonized air. It is known that ozone is a good bactericide and virucide, which improves the organoleptic qualities of water and which, besides its oxidation properties, particularly for iron and manganese, constitutes a good flocculating adjuvant. With an equal quality of settled water, it makes possible a saving of coagulant and, with an equal amount of coagulant, it makes it possible to eliminate dissolved organic materials better. However, ozonization has a disturbing impact when it is followed by a standard settling because of the phenomena of spontaneous flotation in flocculators and settling tanks.
However, in current practice known, the two phases of ozonization and then of flotation are successively used in water treatment installations.